The role of immune response in the development of periodontitis is the subject of this project. As an animal model for these studies, germ-free Sprague-Dawley rats are monoinfected with bacteria. The bacterial strains for this study are selected from those microorganisms which (a) were isolated from periodontitis patients and (b) which were already shown by others to induce periodontitis-like lesions in germ free rats. Measurements of the immune response to the monoinfecting microorganisms are carried out in longitudinal studies and correlation is sought between the immune response and the disease development.